Fuel cells have been proposed as a clean, efficient and environmentally responsible power source for electric vehicles and various other applications. One example of a fuel cell is the Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally comprises a thin, solid polymer membrane-electrolyte having a catalyst and an electrode, for example an anode or a cathode, on both faces of the membrane-electrolyte.
The MEA generally comprises porous conductive materials, also known as gas diffusion media, which form the anode and cathode layers. Fuel, such as hydrogen gas, is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and hydrogen cations. The electrons are conducted from the anode to the cathode through an electrical circuit. Simultaneously, the hydrogen cations pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the hydrogen cations to form water as a reaction product.
The MEA is generally interposed between a pair of electrically conductive contact elements or bipolar plates to complete a single PEM fuel cell. Bipolar plates serve as current collectors for the anode and cathode, and have appropriate flow channels and openings formed therein for distributing the fuel cells gaseous reactants (i.e., the H2 & O2/air) over the surfaces of the respective electrodes. Typically, bipolar plates also include inlet and outlet headers which, when aligned in a fuel cell stack, form internal supply and exhaust manifolds for directing the fuel cells gaseous reactants, water and liquid coolant to and from, respectively, a plurality of anodes and cathodes.
It is known that membranes within a fuel cell need to have a certain relative humidity to maintain an ionic resistance across the membrane within a desired range to effectively conduct protons. Generally, if the humidity is too high, the flow channels can become blocked by an accumulation of liquid water in a phenomenon known as “water stagnation.” Such water stagnation can inhibit or prevent the flow of the gaseous reactants and seriously impair the performance of the fuel cell.
Fuel cell systems of the art can employ hydrogen at a pressure or velocity sufficient to push liquid water out of the system and minimize water stagnation. However, since an anode stoichiometric ratio of about 1.0 to about 1.05 is required for optimum fuel utilization, employing hydrogen at the sufficient pressure or velocity reduces the fuel cell efficiency. To increase anode gas velocity without sacrificing the fuel utilization efficiency, a number of anode architectures have been investigated. These architectures have included flow shifting, stack order switching, and anode recirculation. Flow shifting involves an alternating flow of hydrogen gas through a first stack and a second stack connected in series, wherein the direction of the flow through the stacks alternates. Stack order switching involves an alternating flow of hydrogen gas through a first stack to a second stack connected in series, wherein the direction of flow through the fuel cell stacks remains constant. Anode recirculation involves a recycling of anode exhaust gases back through the anode for consumption of residual hydrogen gas. However, for reasons relating to effectiveness, efficiency, and durability, these individual architectures have proved to provide an insufficient gas velocity.
Nitrogen accumulation on the anode due to cross-over from the cathode can also create localized regions of hydrogen starvation in a phenomenon known as “nitrogen stagnation.” Typically, the nitrogen on the anode will accumulate to an undesirable level as the hydrogen fuel is consumed. This accumulation is a present in all of the anode architectures, necessitating the use of a bleed valve that vents recirculating anode gases before the undesirable nitrogen levels are reached.
There is a continuing need for an anode architecture that will remove accumulating water more effectively and efficiently, and that will involve a simplified fuel cell system. Desirably, the method will include an opportunity to effectively bleed accumulating inert gases from the anode of the fuel cell stack.